San Francisco ? Oakland pot activists fresh off a victory at local polls on the taxing of medical marijuana took their first official step Tuesday toward asking California voters to legalize pot.

A proposed ballot measure filed with the California attorney general’s office would allow adults 21 and over to possess up to an ounce of pot. Homeowners could grow marijuana for personal use on garden plots up to 25 square feet.

The measure’s main backer is Oakland medical marijuana entrepreneur Richard Lee, who helped push a first-of-its-kind tax on city medical marijuana dispensaries that passed with 80 percent of the vote last week.

The statewide measure needs nearly 434,000 signatures to make the November 2010 ballot.

“It’s one more pretty amazing element in the momentum toward ending statewide prohibition,” said Stephen Gutwillig, California director of the pro-legalization Drug Policy Alliance.